1. Field of the Invention
The present invention relates to optical waveguides, and more specifically, it relates to single mode operation of optical waveguides.
2. Description of Related Art
Today the most common fiber structures consist of a single step index that serves to confine radiation as it is propagated along the length of a fiber. By appropriately coordinating the magnitude of the jump in the step index together with the diameter of the laser core, it is possible to ensure only a single transverse mode will be guided by the fiber. FIG. 1 illustrates a prior art fiber structure with a step index 10 and the corresponding irradiance profile of the guided radiation 12.
This is an extremely useful configuration because single transverse mode radiation is very close to being diffraction limited when it leaves the fiber. Diffraction limited light is the highest attainable quality of light for propagating through free space, minimizing diffractive spreading and enabling focusing to the smallest possible spot with the use of a simple lens. The limit on the pulse energy possible in such a waveguide structure is determined by the onset of deleterious nonlinear effects such as stimulated Brillouin scattering, stimulated Raman scattering, self phase modulation, and optical damage to the fiber medium itself. Scaling such structures to larger pulse energy is done by increasing the core area, i.e., the area associated with the step index. Doing this does however require that the height of the step in the index be decreased to ensure the fiber only supports single mode propagation as the core area is grown. The following relation gives the maximum value of the index step, Δn, that will guarantee single mode operation
                              2          ⁢          π                          λ          0                    ⁢      a      ⁢                                    2            ·            n            ·            Δ                    ⁢                                          ⁢          n                      <    2.4    ,where a is the radius of the core. This relation is plotted in FIG. 2A where the maximum value of the step index Δn that is permissible for a given structure to remain single mode is plotted against core diameter for the specific case of λ=1.080 μm and a base index value of n=1.5.
The area 20 in FIG. 2A denotes that portion of the parameter space that is directly accessible by convention single mode fibers. The record core diameter reported today for a step index fiber is 80 μm, although this fiber reportedly had a large Δn value of ˜1.2×10−3, permitting the propagation of many modes beyond the preferred diffraction limited one. The practical limit in going to larger core area fibers that are truly single mode is the increased precision with which refractive index control must be incorporated into the waveguide. Using conventional fiber technology, control on the level of ˜1×10−4 is possible, from which one concludes it will not be possible to push single mode fiber waveguides beyond about 50 microns in core diameter.
Single mode waveguides and fibers having core diameters exceeding that attainable with conventional fiber technology are desirable.